Data compression can be enabled when configuring history collection for a history tag, by enabling data compression and entering a deviation amount. You can also select the type of display representation: Step, Line, or Automatic. Step representation displays the values as step changes on the trend and is usually used for discrete values, such as pumps, and for values that change in a step-like manner, such as setpoints. Line representation displays values with point-to-point connections and is usually used for values such as process data. It is recommended that either Step or Line be specifically selected when configuring the Display Representation and that the plot method used to display the data in Process History View be set to the default setting. If the Display Representation is left as Automatic, then it will set a floating point value to Line, and an integer to Step.
When data compression is disabled for a history tag, a sample value, status, or time stamp is captured in the historical database for each scan cycle, determined by the configured scan period. Even if the value of the history tag remains the same, a sample will be stored with the same value/status for each scan cycle.
When data compression is enabled for a history tag, the only samples stored are those that are needed to represent the historical data record for the tag to the desired degree of accuracy. Clients reading archived data for a compression-enabled history tag should expect the request to return a variable number of unevenly spaced samples. Enabling data compression reduces storage requirements and improves performance when retrieving the historical data record for history tags that change infrequently or by insignificant amounts.
As mentioned above, when configuring the history collection for a tag, you can select a display representation of Step, Line, or Automatic. The display representation chosen during configuration is important for several reasons:
The Process History View application allows you to select a Line or a Step plot method for a given trend, regardless of the display representation configured for the tag’s history collection and regardless of the compression technique used for gathering the data. Mixing the configured display representation and the Process History View trend plot method can result in misleading trend graphing outputs. For example, if a history tag is configured for Line display representation, data collection and compression are optimized for producing sloping line rendering of historical data. If you override the default and selects Step plotting in Process History View, the trend may indicate that changes occurred in abrupt steps, when actually the trended parameter changed gradually at a nearly constant slope. Conversely, if a history tag is configured for Step display representation and you selects Line plotting in Process History View, the trend may indicate the values of a parameter changed gradually when they did not.
In summary, optimal rendering results will be achieved when the trend plot method in Process History View matches the display representation configured for the tag history collection. It is therefore recommended that the default plot method be used in Process History View, rather than selecting a plot method that might conflict with the configured display representation.
Changing the display representation and downloading will affect the subsequent behavior of the system, but it does not change the historical data already collected for a history tag. No attempt is made to track changes of display representation. The current configuration is interpreted as the preferred choice for rendering and interpolating values in all previous historical data stored online for the tag.